Brushless DC motor

Details Brushless DC motor Brushless DC motor

2002-09-17

2004-03-23

Le, Dang (Department: 2834)

Electrical generator or motor structure

Dynamoelectric

Rotary

C310S049030, C310SDIG006

Reexamination Certificate

active

06710504

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a brushless DC motor having a magnet magnetized so as to have a plurality of magnet poles in the circumferential direction thereof for use in a small fun•blower and, more particularly, relates to a brushless DC motor comprising two stator yokes, each having main magnetic pole and magnetic pole pieces, a center yoke passing through the center portions of the two stator yokes for connecting magnetically the two stator yokes, and an annular winding arranged between the two stator yokes.
2. Description of the Prior Art
Japanese Utility Model Laid-Open No. 153486/86, Japanese Patent Laid-Open No. 63065/83, Japanese Patent Laid-Open No. 214759/86, Japanese Patent Laid-Open No. 23754/89, and Japanese Patent Laid-Open No. 303750/94 propose many kinds of the brushless DC motor.
FIG. 8
is an exploded view of the conventional brushless DC motor disclosed in the Japanese Patent Laid-Open No. 23754/89.
FIG. 9
shows a relation between the magnet and the stator yokes.
FIG. 10
is a sectional view of the conventional brushless DC motor.
The brushless DC motor of four poles shown in
FIG. 8
has a rotor
1
consisting of a cylindrical rotor yoke
2
of the magnetic material, a magnet
3
magnetized so as to have a plurality of magnet poles in the circumferential direction thereof, and a motor shaft
4
. Reference numeral
5
denotes a first stator yoke having two main magnetic poles
13
and magnetic pole pieces
21
. Reference numeral
23
denotes a bobbin,
24
denotes an annular winding wound around the bobbin
23
. Reference numeral
25
denotes terminals mounted on the bobbin
23
, to each of the terminals
25
each of lead wires of the winding
24
being tied up and soldered,
6
denotes a second stator yoke having two main magnetic poles
14
and magnetic pole pieces
22
. Reference numeral
7
denotes cylindrical center yokes formed on center portions of the first and second stator yokes
5
and
6
, inserted telescopically each other so as to connect magnetically the first and second stator yokes
5
and
6
each other,
17
denotes a slit formed axially on each of the center yokes
7
,
9
denotes an electromagnetic conversion element, such as a Hall element for detecting the magnet pole of the magnet
3
,
11
denotes a printed wiring board for connecting electrically the Hall element
9
and the terminals
25
to a driving circuit.
FIG. 11
shows a driving circuit for two-phase half-wave electric currents energizing alternately two windings
24
a
and
24
b
having a phase difference of 180° in electrical angle. In
FIG. 11
, reference numeral
60
denotes a motor portion,
61
and
62
denote transistors each driven by a signal from a motor driving IC
63
,
64
denotes a DC power source, and
65
denotes electric parts or IC parts installed on the printing wiring board
11
.
In the Japanese Patent Laid-Open No. 23754/89, on page 4, lines 30-38 and in FIG. 3 thereof, a figure and an arrangement of magnetic pole pieces of first and second stator yokes are disclosed, as shown in
FIG. 9
of the present application.
The motor shown in
FIG. 9
, however, has a dead point at which the motor cannot be driven in principle due to the specific arrangement of the magnet and the magnetic pole pieces. In order to avoid such dead point and to reduce a torque ripple during the rotation of the motor, a relative position of the magnet and the magnetic pole pieces is considered.
Specifically, it is stated in the prior art that it is preferable to reduce a spread angle a of the magnetic pole piece to ⅕ to ⅘ of that of one magnet pole of the magnet
3
, and to set a spread angle b formed between the center lines of the magnetic pole pieces
21
and
22
of the first and second stator yokes
5
and
6
to &pgr;/4 to 3&pgr;/4 in electrical angle.
Further, it is suggested in the publication that it is preferable for a high speed brushless DC motor to provide a slit
17
in the axial direction in each of the center yokes
7
, in order to reduce the eddy current and the eddy current loss produced by the magnetic flux in the axial direction.
In the Japanese Patent Laid-Open No. 303750/94, on page 3, line 34 to page 4, line 12, it is pointed out that the sum of the current torque and the cogging torque becomes the motor torque, and there is a problem in the negative torque and the ripple of the motor torque, and proposed that the spread angles a and c of the magnetic pole pieces of the first and second stator yokes shown in
FIG. 9
of the present application are different from each other. Further, in the Japanese Patent Laid-Open No. 63065/83, it is suggested that a projecting portion
20
is provided on each of magnetic pole pieces of the first and second stator yokes so as to project in a direction reverse to the rotary direction of the magnet different from the above prior art, as shown in
FIG. 12
of the present application.
However, in the conventional motor as mentioned above has following defects.
In the conventional brushless DC motor disclosed in the Japanese Patent Laid-Open No. 23754/89, the winding
24
is wound around the bobbin
23
, the lead wires of the winding
24
are tied up and soldered to the terminals
25
mounted on the printed wiring board
11
, as shown in
FIG. 8
of the present application. In such structure, the cost for the parts, such as the bobbin
23
and the terminals
25
is increased. Further, the treatment of the lead wires of the winding and the soldering thereof are complicated and the cost thereof is increased.
Further, in the above brushless DC motor, it is necessary to use the IC driving circuit for driving the two-phase bifilar windings
24
a
and
24
b
having a phase difference of 180° in electrical angle. The space factor of the winding
24
is reduced remarkably because two wires are jumbled together when the bifilar winding is wound around the bobbin.
Further, the magnetic flux from the magnet cannot be used effectively, because the space angles of the first and second stator yokes are different from each other and thus the spread angle a of the magnetic pole piece becomes small with respect to that of the one magnet pole of the magnet. Further, in the arrangement shown in
FIG. 9
, a magnetic flux from the N pole of the magnet
3
is returned to the S pole of the magnet
3
through the first stator yoke
5
, the center yokes
7
and the second stator yoke
6
. However, the magnetic flux from the N pole to the S pole of the magnet
3
is limited and entire magnetic flux cannot be returned to the S pole of the magnet, because a portion of the magnetic pole piece of the second stator yoke
6
is positioned at the boundary of the N and S poles of the magnet. Further, the effective interlinkage magnetic flux and thus the motor efficiency are reduced, because the magnetic flux passing through the center yokes
7
is interlinkaged with the annular winding.
FIG. 7
is a graph showing relations between the interlinkage magnetic flux of the motor winding and the counter electromotive force with respect to the motor revolution, in case that the materials and figure of the center yoke are varied. In
FIG. 7
, a curve {circle around (1)} shows the counter electromotive force in case that an electromagnetic soft iron is used, and {circle around (4)} shows the interlinkage magnetic flux. The frequency of the alternating magnetic flux passing through the center yoke becomes high and the interlinkage magnetic flux is suppressed by the eddy current according to the increase of the rotation, so that the counter electromotive force is saturated. In
FIG. 7
, a curve {circle around (2)} shows the counter electromotive force and {circle around (5)} shows the interlinkage magnetic flux in case that one slit
17
is formed axially in each of the center yokes
7
as shown in FIG.
8
. By the effect of the reduction of the eddy current, the counter electromotive force and the interlinkage magnetic flux are improved in the high speed rotation region, however, the interlinkage magne

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